This is a proposal to continue the development of radioiodinated tracers for mapping presynaptic nerve terminal density in mammalian brain. Parkinson s disease is characterized by massive degeneration of nigrostriatal dopamine neurons. Radiotracer imaging methods tend to underestimate actual nerve terminal loss possibly because of compensatory responses of surviving nerve terminals. A radiotracer that binds to surviving nerve terminals at a site minimally involved in homeostasis may provide a more precise assessment of nerve terminal loss. This capability would be especially important in tracking the long term efficacy of neuroprotective drug strategies initiated in patients with mild Parkinson s disease. Kilbourn and co-workers have developed a method for determining the regional brain density of the type 2 monoamine vesicular transporter (VMAT2) using [C-11]dihydrotetrabenazine (DTBZ). Studies with DTBZ have shown that striatal VMAT2 density is linearly correlated to the integrity of substantia nigra dopamine neurons and not subject to drug- or lesion-induced compensatory regulation. This proposal will focus on developing radioiodine labeled derivatives of DTBZ that selectively bind to VMAT2. A successful ligand will require simultaneous modification of two areas of the DTBZ molecule - one for iodine attachment and a second for polar group incorporation to counteract the lipophilicity-enhancing effect of iodine. Candidate iodo-DTBZ derivatives will be screened in vitro by radioligand binding assays using rat brain and bovine chromaffin vesicle ghosts. Ex vivo studies in rodents with [I-125]tracer will include determination of regional brain distribution by direct sampling/counting and by autoradiography. Tracer kinetic analysis with determination of brain and blood metabolites will be performed in rats. Validation of VMAT2 binding specificity will include testing for saturability, blockade by DTBZ and reserpine, stereoselectivity, and response to graded 6-hydroxydopamine striatal lesions. The most promising tracers will be labeled with iodine-123 and studied in baboons by emission tomography to test safety, imaging characteristics, and striatal washout time.